Along the line of the wall are situated fortified enclosures of three kinds which now go by the names of camps, mile-castles and turrets. The _camps_, of which there were seventeen, between Tyne and Solway, and which were probably called by the Romans _Prætenturæ_ or _Stationes_, vary in size from three to six acres. They were destined for the housing of one cohort--a body of men varying in size from 600 to 1,000--with, no doubt, a certain number of camp-followers, and in some cases a considerable troop of horses. Public buildings, known by antiquaries as the prætorium, the forum and the like, are to be found generally in the centre of the camp, sometimes on the side most exposed to the enemy’s attacks: and the quarters of the officers may generally be distinguished from those of the common soldiers by the elaborate arrangements for warming them, known as hypocausts. In these the floor of the room is supported on ranges of short pillars (generally about eight or nine inches high), between which the hot air circulated, being brought by flues from the furnace at a corner of the camp, in which it is evident that the fuel used was often the coal of Northumberland. The great number of oyster-shells, the beef-bones and mutton-bones found near many of the camps give us an indication of the food supplied to the officers, perhaps also to some of the privates. Many interesting illustrations of the immense length of time that the Roman occupation of Britain endured may be derived from these _Prætenturæ_. Thus we have several inscriptions recording the repair of a granary or a temple ruined by age (_vetustate conlapsum_): and in the sacred well of the nymph Coventina, just outside the camp of Procolitia, there were found 16,000 coins ranging over a period from A.D. 100 to 300 which had been thrown into the well by generations of Roman soldiers as votive offerings to the goddess. Besides the larger camps, there were, as has been said, also smaller forts, erected at regular intervals of a thousand Roman paces, which are now known by the designation _mile-castles_; and other still smaller enclosures, hardly more than sentry boxes, about three to the mile, which are called, not very aptly, _turrets_, and of which very few specimens still remain.